The invention relates generally to control of the movement of a motor driven machine element or mechanism, and in particular the invention is concerned with an adaptive drive control positioning system which tracks and stores velocity, deceleration and acceleration, both in a current drive cycle and in previous drive cycles, to anticipate the effects of motor drive on machine element position so that drive power can be shut off at a correct point for the machine element to coast to the target position.
A number of motor control systems have been suggested for achieving correct positioning of a driven mechanism or machine element with the objective of minimizing the time to reach the target position. U.S. patents disclosing such systems include Sweeney, Jr. U.S. Pat. Nos. 4,353,019 and 4,571,530, Sweeney U.S. Pat. No. 4,312,033, Davey U.S. Pat. No. 3,917,930, Pearson U.S. Pat. No. 3,723,843, and Higomura U.S. Pat. No. 4,710,865.
Sweeney, Jr. U.S. Pat. No. 4,353,019 discusses an adaptive pulsing motor control for a positioning system wherein the slow down from high speed is distance adaptive only. The final positioning requires a stop between pulses during settling. The stop period is set and relatively long (50 ms) which causes relatively slow settling response. The increased pulse width to the motor until motion has occurred can rock the mechanism loose from a static bind and cause immediate overshoot of the target. Velocity is not allowed to remain greater than zero until the final target is located.
The second Sweeney, Jr. patent is somewhat similar to the first, but includes the concept of "nudging" in the final approach positioning technique. In the "nudging" technique the pulses of the earlier Sweeney, Jr. patent are used but are not adapted significantly. Instead, to allow the computer to control two drives simultaneously, the DC power in the drive control circuit is adjusted.
Sweeney U.S. Pat. No. 4,312,033 disclosed a system employing a fast motor and a slow motor. The slow motor, used to effect the final few inches of movement, was pulsed on and off in the last few inches, to a series of complete stops. Such complete stops, particularly near the target position, are undesirable because of a "flex back" effect, i.e. the tendency of some mechanical components or assemblies to move backward (to relieve strain) immediately after being moved forward.
Davey U.S. Pat. No. 3,917,930 discusses "random" and "long term" errors in a system for adaptively positioning a machine element. These errors are adjusted for within range and step limitations on a block move by block move basis (displacement only, with no velocity evaluation). Positioning is accomplished on the basis of immediate stop and assumed zero coast of the mechanism. Further, to set up the Davey system for operation, a semi-manual pre-staging of the move is required, and all physical factors must stay basically the same for that specific move thereafter.
Pearson U.S. Pat. No. 3,723,843 discloses the use of dynamic breaking of a motor to bring the motor from a slow positioning speed to a stop without adjustment for variations in mechanism response. This would presumably result in poor performance, since it would tend to remove the driven mechanism from the face of the leadscrew and introduce an inherent error beyond the error created from the non-adaptive brake.
The Higomura patent has pertinence to the present invention in that it discloses use of velocity as a locating method. Higomura adapts velocity based on a very tight electronic control circuit which assumes it can adjust virtually immediately to any and all changes. The system dictates to the driven mechanism rather than reacting to the mechanism. There is no velocity profile evaluation and no on-the-fly adjustment of "fall over to target" value. Likewise, there appears to be no adjustment for the high speed deceleration entry point based on the actual current driven speed. Higomura's velocity curve, however, appears somewhat similar to that of the present invention described herein.
Positioning motors have been of three basic types in previous positioning systems: stepper motors, three-phase AC motors and DC motors with servo controllers. Stepper motors usually have up to five windings, with a driver required for each winding. Pure DC current is needed, and every winding must be connected properly. A stepper motor can slip, missing the next desired point. Further, there are limitations on speed, with relatively slow top speeds. In addition, stepper motors and the required controllers tend to be quite expensive.
Three-phase AC motors are relatively inexpensive, but require costly frequency converters. Further, these motors do not have good slow-down capability. Speed is controlled by a frequency controller. The Sweeney '033 patent discussed above employed two motors of this type, one for fast drive and one for slow drive.
In DC motor systems with servo control, the motor itself was not expensive, but a DC supply was required and the servo control systems tended to be large, expensive and complex.
The drive control positioning system of this invention is far more adaptive than any of the devices described in the above patents, and enables a target position of a machine element to be achieved quickly, efficiently and reliably.
In discussion of the problems addressed by the present invention and the features of the invention, certain terms are used herein and are defined as follows.
Delta: The distance remaining to the target position. PA0 Drive Cycle: The half wave (of AC power) during which time the deceleration, motor drive, acceleration, and displacement are monitored by the drive system. PA0 Fall Over Point: The distance from the target at which the driven mechanism will coast (fall) into the target zone by the time the speed diminishes to zero by means of natural deceleration (due to frictional losses). This value is a function of actual speed and anticipated deceleration due to mechanical losses (this is a non-linear curve since it is unlikely that the stored energy within the driven system is directly proportional to the velocity of the mechanism). PA0 Flexback: The tendency of some mechanical assemblies to move backward (to relieve strain) immediately after being moved forward (or vice versa). This is particularly evident in driven mechanisms consisting of nylon, rubber, or other potentially elastic components. These components have their greatest effect when they are located farthest from the drive power source. The slower the actual velocity, generally the greater the effect of flexback. PA0 Ideal Velocity Curve: A curve which results from plugging measured machine data into primary equations (v=at; v=v.sub.cap ; v=A.times.delta.sub.adj. ; v.ltoreq.v.sub.base ; curve A.sub.2 .ltoreq.v&lt;curve A) which are used to define a series of phases of motor drive as described herein (see FIG. 1). The points where these curves cross over each other define the phase change points. This is used initially to determine with which phase to start the drive procedure. Thereafter, actual velocity is compared with this curve to determine when a change in phase is appropriate. Ideal velocity is dynamic and is modified as a positioning cycle progresses. PA0 Ideal Speed: The speed defined by the delta and the ideal velocity curve. PA0 Macroadaptation: The adjustments performed by the invention to evaluate, predict, and overcome the changes in the mechanical drive parameters which occur at higher velocities. This plays a major factor primarily in determining the braking entry point and power adjustment during deceleration during the positioning sequence or routine (see FIG. 1, Phase 3). PA0 Microvariation: The changes in drag, static friction, dynamic friction, and coast which vary dramatically at low velocities. These can be caused by, among other things, localized dirt, irregular surfaces, uneven drive belts (or other transmission devices), out of round or misaligned pulleys, bent shafts or leadscrews, and motor brush cogging. PA0 Microadaptation: The adjustment performed by the new electronics to overcome the effects of microvariation and allow fast, accurate positioning of any driven mechanism independent of the degree of variation of the mechanical parameters. PA0 Pressure Faces: The surfaces of the power transmission devices (gears, wormgears, etc.) which make direct contact between the drive motor and driven mechanism to cause movement toward the target location. It is imperative that all of these surfaces remain substantially in contact when the target is obtained since position is monitored by a shaft encoder which does not measure directly off the driven mechanism or machine element. Generally shaft (i.e. a leadscrew) momentum is greater than momentum of the driven mechanism or machine element on coasting, keeping the pressure faces in contact during coasting to target. PA0 Target: The position which is to be obtained by the driven mechanism. PA0 Target Zone: The region defined by the target value plus or minus the selected settling tolerance. PA0 Tolerance: The amount of acceptable error in positioning when the driven mechanism comes to a final stop.